with the advancement of the optical communication technology, high-speed and high-stability optical modulators have been required. As a high-speed optical modulator, a Mach-Zehnder optical modulator has been known. The Mach-Zehnder optical modulator splits an input light and combines the split light with a phase difference added thereto to obtain an output light of which intensity is modulated.
FIG. 1 shows a conventional Mach-Zehnder optical modulator. The Mach-Zehnder optical modulator 100 comprises, on an optical substrate (of a lithium niobate crystal (LN: LiNbO3) or the like) 110 having an electro-optic effect, an input waveguide 112 for inputting an input light, a Y branching portion 114 for splitting the light from the input waveguide, two arm waveguides 116a, 116b for respectively guiding the split light, a Y combining portion 118 for combining the light from the two arm waveguides, and an output waveguide 120 for outputting the combined light. These waveguides can be formed by selectively diffusing a metal, such as Ti, into the optical substrate. Thereafter, a buffer layer of SiO2 or the like is provided on the entire surface of the substrate, and metal electrodes 122a, 122b of Au or the like are formed on the respective arm waveguides.
The electrodes 122a, 122b are connected to a bias circuit 124 for setting an operating point of the optical modulator and a high-frequency signal source 128 for modulating the optical modulator, and between these electrodes, a terminal resistor 129 is connected. In addition, the bias circuit 124 is connected to a power supply 126 for supplying a DC voltage.
The input light made incident into the input waveguide 112 is split into two at the Y branching portion 114. While propagating through the arm waveguides 116a, 116b, the split light come under the influence of an electro-optic effect caused by a modulating signal applied to the electrodes 122a, 122b and change its phases. In short, the phase difference between the arm waveguides can be changed by the signals applied to the electrodes. When the light from the arm waveguides 116a, 116b are combined at the Y combining portion, light of which intensity is modulated depending on the phase difference between these two light is launched from the output waveguide 120.
FIG. 2 shows the relation between the phase difference of the light and the intensity of the output light. When the voltage applied to the electrodes is zero, no phase change due to the electro-optic effect occurs in the arm waveguides. Accordingly, if the arm waveguides are equal in length, the phase difference becomes zero. In this case, the intensity of the output light having two light from the arm waveguides combined becomes the maximum. By increasing the voltage applied to the electrodes, the phase difference becomes increased. When the phase difference is n, the two light from the arm waveguides are canceled, and the intensity of the output light becomes the minimum.
In practice, in order to maximize the extinction ratio of the output light, the operating point of the optical modulator is set at a middle voltage between the voltage maximizing the intensity of the output light and the voltage minimizing the intensity, and the modulating signal is applied to this operating point (PTL 1). The operating point may be set by applying a DC bias (from the bias circuit 124 in FIG. 1) between the electrodes in addition to the modulating signal (from the high-frequency signal source 128 in FIG. 1). Alternatively, as shown in FIG. 3, heaters 342a and 342b may be provided on the arm waveguides in addition to the phase modulating electrodes 322a and 322b. When the waveguides are heated by the current sources 344a and 344b with the heaters 342a and 342b, a phase difference between the arm waveguides because of a thermo-optic effect is caused, which can set the operating point (PTL 2). In this case, between the phase modulating electrodes 322a and 322b, the modulating signal may be applied directly from the high-frequency signal source 328 without applying the DC bias.
However, the conventional method of setting the operating point of a Mach-Zehnder optical modulator has problems described below. First, in the method of setting the operating point with a DC bias, if the DC bias is applied for a long period of time, the operating point changes with time (DC drift phenomenon), resulting in a problem of degrading the modulation characteristic. Therefore, it is necessary to monitor the output light of the optical modulator and to provide feedback so as to adjust the voltage of the DC bias. In addition, considering 20-year of use, as an adjusting range of the DC bias, a voltage source having a wide variable range of approximately ±15 V or more is required.
In the method of setting the operating point with heaters provided on the arm waveguides, if an optical substrate of a ferroelectric, such as LN, is heated, an electric field is generated in the substrate by polarization depending on the temperature. This electric field causes an unnecessary phase change (thermal drift) in the waveguides, resulting in a problem of fluctuating the operating point. Moreover, if the ferroelectric substrate is heated, deformation in the substrate is caused with heat, and the operating point is destabilized with the piezoelectric effect. In the worse case, the substrate may be broken by static electricity charged on the surface of the substrate. Furthermore, the substrate may be broken by thermal expansion. In particular, in order to improve high frequency characteristic, it is necessary to reduce a thickness of the substrate (to a thickness of approximately 0.25 mm), which is more likely to cause such breakage. Even if the substrate does not break, warpage may occur in the substrate, which causes displacement with optical fibers at the input and output portions of the substrate, resulting in degradation in insertion loss and increase in return loss. On the other hand, in order to maintain a desired phase difference, a certain temperature gradient continues to be provided between the waveguides. However, the LN substrate is a crystal and thus has a relatively high thermal conductivity (approximately 5 W/(m·K)), which causes heat to diffuse over the entire substrate, and the temperature of the substrate tends to be equalized. Therefore, there are problems in that it is difficult to stabilize the operating point and that power consumption becomes high.
The present invention has been made in view of such problems, and has an object to provide an optical modulator having a high stability.